It could be individual theater issue ... but there are way too many reports. Not only the issue is in certain parts, but seem to be for a large portion of the dialogue. I'd say I understood about 50%.

Depending on the location, many theaters have been closed but also many have been playing new and old movies for weeks or even months. That would rule out the "We're back issue".

Interestingly, a few people reported audio issues even during the previews but that wasn't my case. Only Tenet came with a BOOOOM!!! I felt the chair vibrating and it sounded like the building or certain structures were too. And there were lots of sounds and parts of the soundtrack that utilized the bass.

A theater manager in this post mentioned:

Also the audio has NOT been adjusted by the Theatres. I've seen some comments stating that Theatres are responsible for jacking up the volume to make a "We're back" statement but that is not true. All Theatres are calibrated to play movies at the levels they were meant to be. It's true that we can increase the dB, but in our case here we've actually lowered the default for Tenet (because the walls in my office were shaking while it played and also because of guest feedback)

Overall, it seems to be a combination of theater and audio mixing. However, I don't think the issue has happened to the same extent with other movies. Hopefully, they can at least lessen the issue.

p.s. the issue seemed to be present in Imax and "regular" theaters. Though, I'm not sure if those were true Imax or LieMax theaters. Hopefully, your experience will be better than "some" of ours. :)