We don’t support Ubuntu 18.04 LTS, our final notice was this back in late April 2021, or three years after we released Lubuntu 18.04 LTS.
Ubuntu support sites can be found here.
The only currently supported products of Lubuntu are Lubuntu 20.04 LTS (with mere weeks of support only remaining), Lubuntu 22.04 LTS, and Lubuntu 22.10.
In your situation, I’d explore what kernel stack you’re running for starters (I mentioned the two primary stacks (GA & HWE) earlier) and see if that changed as I alluded to with 22.04 LTS earlier on this thread; Note: the last stack change occurring when bionic reached 18.04.5; with the 18.04.6 change relating to key revocation. But I won’t support a bionic system here sorry.