Dictionaries do not tend to go into very much detail about the variation in pronunciation about different words. To get this information you need to go to a really good dictionary or, even better, a really good pronunciation dictionary.
Without any shadow of a doubt, the recognised leader in this field is the Longman Pronunciation Dictionary, which will give you good accurate detailed information about Southern Standard British English and General American. This information will never be wholly complete. However, for the word demonstrative they give (or rather John Wells the famous phonetician who compiled the dictionary from real data gives):
They don't have any transcription with the stress on the third syllable as in the word demonstrate.
Very often the stress in a word will change according to the suffixes it is given. This will often change the vowels. The penultimate syllable is rarely stressed in adjectives ending in -ative. However, verbs ending in ate usually have some kind of stress on the last syllable in standard Englishes.
Nonetheless, there are always variations between speakers and between different varieties of a language. It's entirely possible that there are speakers of either Gen Am or SSBE who pronounce words like demonstrative with the same stress on the trat syllable as they have in the verb, and with the same vowel too.
What this means is that this may be an idiosyncracy of the speaker, or it may be a variation in the pronunciation used by a minority of speakers, or it may be a pronunciation used by speakers of a particular variety of English. Any which way, it's all good. Variation is good!