Technical background information about O2 sensors:
https://www.hella.com/techworld/ae/Technical/Sensors-and-actuators/Test-lambda-sensor-4379/
The (US)Carreras got heated voltage jump probes in general. If they go bad the signal amplitude gets more narrow and in lower frequency than before.
When the signal is missing for the Motronic / L-Jetronic ECU it will fall back in some kind of a limb home mode. Yes, for engine protection it will run a bit richer.
But: As the Carreras run in general way more lean than the earlier Bosch CIS cars the spark plugs are more lighter / greyish compared to the CIS spark plugs which were more dark / brownish. I camnot say if a missing O2 signal will also cause a more brownish colouring of them or not.
I would check the exhaust manifold temperatures with a IR temp gun to see if a cylinder runs hotter or colder than others. Hotter means it's running (too) lean(er). Then I would measure the O2 probe signal if it's oscillating between 0,1 & 0,9volts with a 1-2Hz frequency.
Cold engine is not optimal to get reliable leak down results.
Before going nuts I personally would fill an injection cleaner additive in the gas tank and go for a longer relaxed spin of at least 1-2hours to get the residues burned. If a valve doesn't close properly on a cold engine due to carbon residues this often may disappear when heating up and driving some time with medium load to get the car back to life, even when it sat for a long time (6months+). This cleaner may help clogged injectors to get them free and improve the spray pattern. This is one of the common reasons for running issues on Carreras.
If the issue then persists then you know that it might be more serious...
And not to forget the mixture switch at the ECU: Make sure that it is set correct for your engine. More details:
https://www.swchips.com/fuelsys.html
Thomas