I recommend using a g4560 or better CPU when using more than 7 GPUs. I would get a g4560 or g4600 and swap it with one of your celerons in your 1st or 3rd rig. If this solves the problem; I would upgrade all your rigs CPUs.
What is the concern running Celeron CPUs with 8+ GPUs?
I have 4 rigs with Celeron CPUs atm as follows:
1. MSI Z270-A PRO, G3950, 6x GPUs
2. MSI Z270-A PRO, G3950, 7x GPUs
3. Asus PRIME Z270-A, G3930, 8x GPUs (soon to be 9x GPUs)
4. Asus PRIME Z270-AR, G3930, 9x GPUs (when they show up)
The first 3 rigs are mining ZEC using EWBF and rig #3 with the 8x GPUs will run for weeks at a time with no issues.
Is the issue with Celeron's only with certain algos and miners?
EWBF doesn't have CPU usage spikes so you will probably be fine with the celerons mining ZEC, however if you mine a Ethash algo coin you will most likely run into problems, more if you dual mine. If you have no issues with the rigs then I wouldn't worry about it; IMO it is best to build rigs with components that are less likely to have problems. For this reason I am currently building rigs using g4600 or g4560 CPUs with the H110 13x motherboards.