The test_incr_backup program detected a problem with an incremental back. I am able to reproduce on current develop, changeset 1d7352fc, which includes the fixes for WT-6136. The command to reproduce and the error:
[sue@ip-172-30-0-18 (1) csuite]$ ./test_incr_backup -S 4333470278817118738 Seed: 4333470278817118738 [1589470630:885071][13133:0x7f6633e42740], file:t10-1.wt, WT_CURSOR.next: __wt_block_read_off, 283: t10-1.wt: read checksum error for 20480B block at offset 3219456: block header checksum of 0 doesn't match expected checksum of 0xa60a9da7 ...