With my understanding the "recovery image" for Pro1 comes together with "boot image". The "boot" is only missing in the naming of the recovery-file offered for LineageOS. The fastboot command flashes the "recovery" into "boot(_a)".
That means, as long the LineageOS-recovery.img is flashed into slot A, the boot-img-part is not the right one for Ubuntu Touch (that also resides in slot A) and Ubutnu Touch wont boot up fully. It needs to have its boot.img (re)flashed into slot A.
So instead of "keeping" I suggest temporarely flashing the LineageOS-recovery-image to slot A, boot into that slo