I remembered it right and you are right too. This is probably a bug.
As a test, I loaded a small a test library and enabled Auto-Import with all bells and whistless. After running Auto-import once manually (to make sure that all old files are fully processed) I dropped a new FLAC album folder into the configured import folder. MC imported the audio files, got cover art from Internet (set to store only externally), and eventually analyzed the files for Replay Gain. MC wrote its own Tool Name and Tool Version tags (probably when Image File field value was saved), but it never wrote the Replain Gain, Peak Level, BPM and Intensity tags to the files.
In addition to not writing the RG tags, the analyzer started only after several minutes had passed and the process was extremely slow. The computer was sitting almost idle while I waited the analysis process to start. It is good that the analyzer does not consume all available CPU when it runs in background, but it should start immediately and run faster when the computer has nothing else to do.
A log file is attached.
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