I haven't used Avamar, but if the log file is growing, either you have a growing workload that needs more log space, or the log isn't being marked as backed up.

An hourly backup (diff/incremental) from Avamar, doesn't mean that it is performing a SQL Server log backup properly. It needs to be marked in msdb.

Have you checked MSDB to see if the log backups are being noted by SQL Server? If not, it's not doing them correctly.