Currently, group membership changes in Special:Log/rights do not show the "User:" prefix, but the "Autopromote block" and "Autopromote restore" logs still do. For consistency, we should also hide the "User:" prefix for those two subtypes.
Examples:
https://en.wikipedia.org/wiki/Special:Log?type=rights&user=&page=&wpdate=&tagfilter=&subtype=blockautopromote
https://en.wikipedia.org/wiki/Special:Log?type=rights&user=&page=&wpdate=&tagfilter=&subtype=restoreautopromote
See also T247174, a similar task for the global rights log.