Our users are also having the same issue from several days. Today we received a report from a user with more details. The device was Samsung Galaxy A51 with Android 10. The user said that the ad was "larger than the screen" and the close button was pushed off the screen. Seems that the issue indeed is the font size. When for our test devices we set the font size to maximum in the phone settings we started seeing broken ads --- the install button was half outside the screen and the layout of the ad was broken. But we still were able to see the close button and close the ad, so we were not able to reproduce the exact same problem on our device to send you a Charles log. As a side note, seems that Samsung devices allow you to set the font to a larger size than most other devices (at least ones we have).
The back button of the device is not closing the interstitals on all our test devices - Samsung Galaxy S10, Samsung Galaxy S9, Google Pixel, Motorola E6 Play, Nokia 1 Plus, Nexus 7 and other. It is also not closing it on the emulator (Google Pixel, Android 10).
We use com.google.android.gms:play-services-ads:19.1.0