Hmm. Indeed there’s something strange. I have tried two procedures: the one on the Lineage Installation wiki on this forum, and the one on Lineage’s own FP3 installation wiki. In the Lineage wiki procedure, the fastboot flash boot <recovery_filename>.img instruction fails (my mac’s Terminal reports “fastboot: error: boot partition is smaller than boot image”).
The equivalent step on this forum, when I send fastboot boot <recovery_filename>.img seems to work, terminal reports:
Sending 'boot.img' (65536 KB) OKAY [ 1.443s]
Booting OKAY [ 39.140s]
Finished. Total time: 40.783s
After that, the phone restarts after a security warning about the unlocked bootloader (with a possibilty to interrupt the reboot), but then doesn’t go into the recovery, but in an ordinary launch, launching /e/.
I can launch recovery from a bootloader restart (Power + Vol-), and then choose to launch recovery, and that lands me in /e/'s recovery. (I had installed /e/ through their “easy install” utility, so I hadn’t met theses difficulties before).
But I never achieved a reboot into an alternate recovery, tried both procedures, and two recovery images (the one from Lineage or the one from Lineage+MicroG, which is my actual goal).