Hello,
Compact mode doesn't remove pure wildcard parameter, while it adds additional parameter as soon as their score becomes 100%. You can also look at https://techdocs.f5.com/kb/en-us/products/big-ip_asm/manuals/product/asm-implementations-13-1-0/24.html for more details.
In general it based on next principle - if you get a lot of traffic with the same parameter, then such parameter will be added to policy as explicit, but if you get a lot of different parameters, then such parameters will be "union" into pure wildcard parameter (pure wildcard parameter will be adjusted to their values).
If your pure wildcard parameter was changed, then this means that you get a lot of different parameters and don't get a lot of traffic with the same parameter.
Thanks, Ivan