There is an /nt/-weakening rule; it's unclear if the /t/ is really "dropped"
To me, there doesn't seem to be anything special about "grunting" vs. "internet", "winter", "printer".
I think the reason for the varied responses in the comments to your question is that native speakers often don't perceive /t/ to be dropped in any of these words (as Teacher KSHuang says). It's undeniable that something often goes on that sounds a lot like dropping the /t/, but as Willow Rex suggests, it may in fact be some kind of "weakening" of the /t/ without it being lost entirely.
Native speaker intuition is not a reliable guide to phonetic details like this, and I don't know what phonetics studies have found about this topic (either option is possible from a theoretical standpoint). So I'll be agnostic on this point and just refer to "weakening" of the cluster /nt/ in this post. I know in the literature, weak /nt/ is sometimes transcribed using [ɾ̃], the IPA symbol for a nasalized alveolar tap.
The /nt/-weakening rule applies in basically the same environments as the more important /t/-weakening rule that exists for most North American speakers. In case you are not familiar with the environment of this rule, it generally applies to a /t/ that is both
- preceded by a vowel or /r/, and
- followed by a reduced vowel (or a vowel in a fully unstressed syllable)
When /t/ is followed by a syllabic consonant such as [l̩], [r̩], [m̩] or [n̩], there is a bit of dialectal variation. Some speakers use weakened /t/ before any of these, but other speakers only weaken /t/ before [l̩], [r̩], [m̩], and use voiceless /t/ (possibly debuccalized to a glottal stop [ʔ], or with strong glottal reinforcement) before [n̩], in words like "kitten" or
The /nt/-weakening rule is optional, unlike the /t/-weakening rule
An important point about "weakening" of /nt/ is that for many (perhaps all) speakers, the /nt/-weakening rule is almost always optional, unlike the /t/-weakening rule, which is for many people obligatory.
For example, I would never pronounce “better” with an unvoiced /t/ sound except in markedly unnatural contexts: trying to imitate another accent, over-enunciating to emphasize a word, or perhaps in singing. I cannot imagine using [t] in "better" in normal running speech, even when speaking slowly or in a formal context (e.g. public speaking or reading aloud).
But it doesn't feel odd to me to use voiceless, un-weakened [t] in the word "center".
My intuition is that there are no absolutely "immune" words
As long as you restrict the rule to its proper environment (e.g. it can't apply before a stressed syllable, as in "entire") I can't think of any word that seems to me to be entirely immune to the /nt/-weakening rule.
There are two "special" words spelled with “nt” in my accent: ninety and seventy. For some reason, I seem to pronounce these as /naɪndi/ and /sɛvn̩di/, with a voiced plosive /d/. I don’t think I use the weakened "nt" sound of "center" in these words ever, if at all, and unweakened "nt" feels odd to me as well.
In contrast, I usually have weakening in the word twenty, but a pronunciation with voiceless [t] is also possible for me in careful speech.
In all other words I can think of with "nt", including "grunting", I feel weakening would be possible for me (although, as I said, not obligatory). But I have no idea how frequent it would be. This kind of thing is very hard to estimate. I certainly feel like I use weakened pronunciations more or less frequently depending on the word, but I don't record my speech so I have no way of quantifying or even verifying this intuition.
I tried to find some data by looking through 39 "YouGlish" pronunciations of "grunting" in American English.
Using this resource, I found two YouTube videos that seem to me to exhibit "weakened" /nt/ in "grunting". Both of them are actually from the same channel, and the man saying "grunting" also seems to have a somewhat non-standard accent in some other respects. (His accent is certainly understandable and not especially strong, but in stangdon's words, it's not what would be considered "correct" or recommended to students of English).
Most of the other 39 YouGlish videos I watched clearly used voiceless [t] in “grunting”. This is a fairly small data set, and it's unfortunately biased by the fact that many of the YouGlish samples came from audio books rather than spontaneous speech, or even prepared public speech.
But based on this data, I would say "grunting" can in fact have weakened /nt/, but it is certainly not mandatory, and in fact it seems to me it would likely be perceived as non-standard (since I've only found two examples of it being used, and in both cases it was used by a speaker who didn't seem to be using a standard English accent).