In an answer to this question, "I hope you enjoyed..." vs "I hope you have enjoyed...", the following is stated:
Both tenses refer to an event that occurred in the past (the enjoyment of whatever is being referred to). However, the first construction focuses more on what happened at the time, whereas the second focuses on the effect of what happened. In other words, the hope being expressed in the first phrase is that the listener enjoyed (whatever is being referred to) at the time it was happening. In the second, the hope is that the listener is currently in a state of having enjoyed whatever is being referred to. The events that would lead to either condition are identical, but the focus is (slightly) different.
At the end of this Cambridge Dictionary blog post about idioms, written by a native speaker of British English, I've encountered a similar construction (emphasis not mine):
[...] I’ll finish with a positive phrase. If someone makes a good offer or suggestion, especially if they have offered or suggested something less attractive before, you can show your enthusiasm by saying Now you’re talking!:
10% more money and my own office? Now you’re talking!
I hope you found these phrases useful. Look out for my next post, on phrases containing the verbs ‘speak’ and ‘say’.
I find this usage confusing. To my mind this appears to be a textbook example of what the quoted answer above says about the usage of "I hope you have enjoyed", repeated once again here:
the hope is that the listener is currently in a state of having enjoyed whatever is being referred to
If I were writing a short piece of text like that blog post, I would expect the reader to have absorbed everything by the end, and that everything is still in their mind, their thoughts. The entire post isn't in the past; it's still being talked about (until the last word) and thought about (perhaps even hours after reading it).
I would therefore hope the reader has found it useful, not that they found it useful 3 minutes ago when they came across an idiom they hadn't known and now no longer find it useful. The goal would definitely be that the reader retains some of the newly found information.
...or is this really the case where it's interchangeable, with no appreciable difference, perhaps even due to the fact it's such a common thing to say ("I hope you found it useful/helpful/etc.")?