After scheduling more pipeline re-builds than we have agents, I see some of those builds are "kind of" started and in fact waiting for executor matching the needed label, but in the legacy UI Stage View optimistically claiming to be "almost complete" (I suppose by comparing the job run time with previous builds). During this display the pipeline engine probably knows this job is not going anywhere, and a tooltip for Estimated time remaining says "N/A".

Obtained Jenkinsfile from 15086f68588c1f9725ec0e0af44481a82fd7b0c9
Running in Durability level: PERFORMANCE_OPTIMIZED
[Pipeline] Start of Pipeline
[Pipeline] node
Still waiting to schedule task
Waiting for next available executor on ‘devel-image&&x86_64’

A picture is worth a thousand words, attached.

I suppose the long blue bar should in fact convey that the build is waiting for executors, or delayed somehow, not that it is "almost complete".