ISI preload online resulting in pre-loaded stimuli not to be displayed

URL of experiment:

Description of the problem:

I have an experiment that I collected data on after plenty of testing/debugging etc. One of my routines contains a blank 300ms preload window, followed by the display of two images stimuli. Both the onset (400-800ms) and the duration (600-1000ms) of those two stimuli are jittered and the pre-loading seemed to give me the most accurate reflection of those times when using manual time-stamps at the beginning and end of a routine.

The whole trial was set up in separate routines to keep the jittered stimuli separate from everything else: fixation (400ms) > blank + preloaded stimuli (min 900, max 1800ms) > stimuli (600ms) > stimuli + response (up to 2000ms) > (feedback if no response, 1000ms)

I collected just over 40 participants on this experiment and made some minor adjustments to the code (unrelated to the above problem) and did a quick test on the code. To my surprise and confusion, the image stimuli do NOT display anymore, instead it is just a long blank period. I would love to blame pandemic fatigue and oversight on this and that when testing my code I simply didn’t notice that those images did not display but I am very sure that those stimuli were shown on the screen before.

Things display fine locally using the pre-loading method. I am using Psychopy 2020.1.3 and Chrome having tested things both on Mac High Sierra and Windows 10.

Any idea or insight whether something has changed in the following time periods:

Data tests before November 9th.
Data collection November 9th to 13th.
Noticed change in display November 20th

My main problem is that I cannot be certain what my participants saw. Did they see those stimuli or not? If yes great, if no not great 42 datasets into the bin.
I have tried to figure things out from the .log files but am none the wiser so far. Any thoughts or help would be greatly appreciated