It was the same with Gradle 7.0.3 plugin 7.0.2 with Android Studio 2020.3.1 Patch 3.
Now with Android Studio 2021.2.1 Patch 1, gradle 7.3.3 plugin 7.2.1, still no debug symbols in app bundles.
If I revert to gradle 4.3.1 plugin 6.5.1, everything is working again.
Noticeably lib folder is also 4 times bigger with latest gradle than it was before!
The gradle builds are not changed in any ways between version of gradle's. One version works (6.5.1), the other does not (7.2.1).
See attached screenshot showing app bundles with plugin 7.2.1 on the left, 6.5.1 on the right.
The left one has no debug symbols included and lib folder is 2.8MB, the one on the right have debug symbols and lib folder is 714KB!
Any help would be much appreciated as it's been 6 months (if not a couple of years at it started after release 6.5.1) I've reported this to Google and received no reply or help whatsoever except 'the debug symbols are generated'. Note that they never said symbols are included in bundles.