It seems it is currently not possible to add new statements with the math datatype. Because of some still unknown error, the js frontend is using {"value":"a + b","type":"unknown"} in its requests, both for wbformatvalue and wbsetclaim.
This might have started with -wmf.11, but it could also be that this is when T266493 started to happen.
Editing the value (in the existing statement) is not possible as well.
Reproducing the issue: edit https://test.wikidata.org/wiki/Q516#P717 and try to save the changed value. Observe the error preventing saving of the new value.
The reason seems to be this change:
T266496#6584787
Notes:
- For local reproduction Math extension needs to be installed/enabled
- It has been speculated the issue might have been related to some broken "wiring" of data types
Acceptance criteria
- it is possible to add statements with the math data type again