Error message: Undefined offset: 51

I’m getting this error message in a couple of templates and I can’t figure out if I’m doing something wrong:

Notice Undefined offset: 51 ee/legacy/libraries/Grid_parser.php, line 139 • Severity: E_NOTICE Notice Undefined offset: 51 ee/legacy/libraries/Grid_parser.php, line 407 • Severity: E_NOTICE

Those templates present a single entry (defined by entry_id) with three grid fields, among others. The templates work fine until I added the third grid field. All the fields both in and outside the grids are pretty simple, text and RTE fields. This happens with two different entries.

I updated to EE 4.3.2 and the error persists.