can't change VT after starting X in lubuntu

somedays this bug just doesn't show up, and other days this sequence seems to reliably capture it:

boot up a fully updated lubuntu 20.04 system (into sddm).
press ctl-alt-F2.
login (on the VT).
startx /bin/x-terminal-emulator

thereafter neither ctl-alt-F1 nor 'chvt 1' seem able to change back to sddm, tho after a rather long time, or some other change in circumstances, it works again.

this bug shows up in a fully updated lubuntu system as long as nobody has yet logged in at sddm.

this bug shows up on the lubuntu iso (wherein sddm has already auto-logged-in to 'lubuntu').

this bug does not seem to show up on a regular lubuntu system if someone is slready logged in at sddm.

this bug does not seem to show up in kubuntu regardless whether anyone is logged in.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: sddm 0.18.1-1ubuntu2
ProcVersionSignature: Ubuntu 5.11.0-37.41~20.04.2-generic 5.11.22
Uname: Linux 5.11.0-37-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.20
Architecture: amd64
CasperMD5CheckResult: skip
Date: Sat Oct 16 21:09:14 2021
InstallationDate: Installed on 2021-09-30 (16 days ago)
InstallationMedia: Lubuntu 20.04.3 LTS "Focal Fossa" - Release amd64 (20210819)
SourcePackage: sddm
UpgradeStatus: No upgrade log present (probably fresh install)