Hi, Jonathan.

Don`t know if the issue is still present for you. But we encountered the same one after upgrade from SQL Server 2017 to SQL Server 2019.
As it seems, the newer version has more aggressive log truncation mechanism.
In reference to this link - https://kb.msp360.com/standalone-backup/ms-sql-server/lsn-do-not-match:
it says: "Two or More Backup Plans That Back Up the Same MS SQL Server instance" may be the issue.

But they forgot to specify, that full and differential backups, that run in parallel can also be considered as two plans.

So, the fix for us was to disable differential backups schedule during the time of full backups. For example, if you run full at 1AM, and it takes 35 minutes to complete, it WILL make three differential backups at 1AM, 1.15AM and 1.30AM (for every 15 min) and mess up those LSN numbers.

Hope that helps.