Use [?] for an incomprehensible lyric.
This is an exact recognized unknown marker, so replacing only the marker is mechanically safe.
Example
[Verse]
I heard (?)[Verse]
I heard [?]The fix
In the workbench this finding carries one control, labelled Replace with [?]. The fix is classified as safe, so pressing it applies the edit directly — one press, one undo step.
Source
The guideline this rule enforces, as cited on every finding it reports:
Unknown lyric marker verified Use [?] for unknown lyrics