TB 8.5a6 using obfs4 did not leave dormant mode on request

Today a long running TB 8.5a6 using obfs4 repeatedly logged about being in dormant mode and did not build any circuits for existing and new tabs until the bridge was disabled. Then everything worked again.

Tor version: 0.3.5.5-alpha (git-a2ecc19a) running on Linux with Libevent 2.1.8-stable, OpenSSL 1.0.2q, Zlib 1.2.8, Liblzma N/A, and Libzstd N/A.

Related: legacy/trac#2149 (moved), legacy/trac#25375 (moved), legacy/trac#28335 (moved), legacy/trac#28847 (moved), legacy/trac#28849 (moved)

Steps to reproduce:

1. Start a fresh TB 8.5a6 with --debug --log and enable obfs4.
2. Use it for some days.
3. Let it run idle for some hours. Step 2 may be optional, the computer was suspended several times but not the day this bug happened.

Will add the exact message when it happens again.