If we verify the WiredTiger.wt file, we're leaving pages in the cache. Here's a simple example, using the thread test program, but I suspect this isn't connected to that program in any way.
bengal:build_posix {41} test/thread/t -1
t: process 29965
1: 10 readers, 10 writers
read thread 0 starting: tid: 0x7fd9ab34f700
read thread 1 starting: tid: 0x7fd9aa94e700
read thread 2 starting: tid: 0x7fd9a9f4d700
read thread 3 starting: tid: 0x7fd9a3fff700
read thread 4 starting: tid: 0x7fd9a954c700
read thread 7 starting: tid: 0x7fd9a2bfd700
read thread 5 starting: tid: 0x7fd9a8b4b700
read thread 8 starting: tid: 0x7fd9a21fc700
read thread 9 starting: tid: 0x7fd9a17fb700
read thread 6 starting: tid: 0x7fd9a35fe700
write thread 10 starting: tid: 0x7fd9a0dfa700
write thread 12 starting: tid: 0x7fd98f5fe700
write thread 11 starting: tid: 0x7fd98ffff700
write thread 14 starting: tid: 0x7fd98e1fc700
write thread 15 starting: tid: 0x7fd98d7fb700
write thread 13 starting: tid: 0x7fd98ebfd700
write thread 16 starting: tid: 0x7fd98cdfa700
write thread 17 starting: tid: 0x7fd983fff700
write thread 19 starting: tid: 0x7fd982bfd700
write thread 18 starting: tid: 0x7fd9835fe700
timer: 2.15 seconds (93023 ops/second)
0: read: 10000, remove: 0, update: 0
1: read: 10000, remove: 0, update: 0
2: read: 10000, remove: 0, update: 0
3: read: 10000, remove: 0, update: 0
4: read: 10000, remove: 0, update: 0
5: read: 10000, remove: 0, update: 0
6: read: 10000, remove: 0, update: 0
7: read: 10000, remove: 0, update: 0
8: read: 10000, remove: 0, update: 0
9: read: 10000, remove: 0, update: 0
10: read: 0, remove: 2019, update: 7981
11: read: 0, remove: 2035, update: 7965
12: read: 0, remove: 2065, update: 7935
13: read: 0, remove: 1989, update: 8011
14: read: 0, remove: 1922, update: 8078
15: read: 0, remove: 2039, update: 7961
16: read: 0, remove: 1994, update: 8006
17: read: 0, remove: 2020, update: 7980
18: read: 0, remove: 1974, update: 8026
19: read: 0, remove: 2012, update: 7988
bengal:build_posix {42} ./wt verify file:WiredTiger.wt
cache server: exiting with 1 pages, 1051 bytes in use
bengal:build_posix {43}