Technically a regression in [0] although it matches behavior prior
to v3.3 going back to 2.7x. The fix for #101883 [1] added a check
for the vertex number changing after the number was zeroed,
causing the shape key to be cleared in most cases.
While the handling of shape-keys in OBJECT_OT_convert wasn't well
defined - clearing the shape-key means in the evaluated coordinates
are always used so it is preferable.
Now the operator ensures the old (un-evaluated) shape-key isn't used.
[0]: 0791f53029
[1]: be32882e1c
b68ba3dae8