tool.drcacheoff.invariant_checker failed with "Syscall instruction not followed by syscall marker"

Not only did the test fail once but 3x in a row:

https://github.com/DynamoRIO/dynamorio/actions/runs/5573200427/jobs/10180157101?pr=6209#step:7:26203
```
Trace invariant failure in T25921 at ref # 86469: Syscall instruction not followed by syscall marker
Test #388: code_api|tool.drcacheoff.invariant_checker .......................***Failed 2.13 sec

Trace invariant failure in T25944 at ref # 86469: Syscall instruction not followed by syscall marker
Test #388: code_api|tool.drcacheoff.invariant_checker .......................***Failed 2.08 sec

Trace invariant failure in T25964 at ref # 86469: Syscall instruction not followed by syscall marker
Test #388: code_api|tool.drcacheoff.invariant_checker .......................***Failed 2.13 sec
```

At first I thought this might be #6105/#6128 but the 3x in a row seems too unlikely for a rare event like that in this test app?