Navien 240A tankless won't feed one faucet

Hello. Not sure if this a good forum for my issue, but here goes. We own a log cabin that does have unusual plumbing because of the construction plans. Basically, the two showers (which sit one on top the other; upstairs/downstairs) are "connected", fed from the cellar. Same for the sinks in both bathrooms, but are separate from the shower feed. That is, running hot water at the sink next to either shower doesn't make the water hotter faster at the showers. But I digress.

We've had the tankless since about 2020, and have had no issues with it. We also have the NaviLink, so I can see what it's doing in real time. Over the past several months, I've noticed that the upstairs shower spmetimes never gets hot. Not even Luke warm. We have to run it a long time regardless as it is the farthest faucet from the Navien. But lately, it'll run five+ minutes and still be ice cold. I've noticed a few patterns:

1. Sometimes while the shower is running to warm up, I've noticed the water cut off, then come right back on. Like the main cutoff outside was suddenly turned off and back on. At first I didn't recognize this other than maybe something the water company was doing.

2. When I watch the Navilink, with this shower running, it shows no BTU usage or any flow. The outflow temperature more or less matches the inflow temperature. No other faucets in the house exhibit this. Wife, who uses the downstairs shower every morning, has never had a hot water issue.

3. It seems that turning the shower from on/off/back on finally kicks the tankless on. Also, it's possible (though I'm still testing) that doing the same thing at the upstairs sink faucet will also kick it on. This makes sense as I've never had hot water issues at the sinks, but this seems to then bring hot water to the shower in question. No other faucets in the cabin have ever briefly stopped flowing water. Just the upstairs shower. I have no idea what would cause a momentary gap in flow.

4. Tankless is in good shape, routinely descaled, and never shows any errors in the Navilink app.

Question: it it possible I have a failing cartridge in the upper shower? I'm almost positive having watched the app that even when it is at full hot on the handle, the app shows no hot water flow. BTU use, or anything to indicate it's working. Yet opening a sink faucet and it immedialy shows activity. I think I've even been able, as stated above, to get it to work but shutting off the shower and turning it immediately back on. Doesn't happen all the time, but enough now so that I am fairly confident it's just this one faucet at that shower. Only thing I can think is the shower, even all the way to hot, is somehow just pulling cold water, hence not turing the Navien "on"? Is this possible? It's been happening over the past six months and it's taken me that long to whittle down what's happening and when.

Any suggestions?

Thanks! C