I forgot about it being inside a case. It's up to you whether the problem is important enough to take apart the case or not...
You say the left decimal is lit so now I'm confused. IN-8-2 are not supposed to have a left decimal - right side decimal only. However, IN-14 has decimals on both sides. Even with IN-14 the clock can only control right side decimals. If you were to pull a working tube out of the clock you would find that the left decimal pin is not connected to any of the pins that plug into the clock board.
Is this the same tube that was giving other problems a few weeks ago? I am wondering if your 10h tube has an internal short. Of course I can't tell unless you swap it to a different position.
As for why the decimal doesn't always light up, that could be dimming related. The dimmer it gets the harder it is for the cathodes to "ignite" and stay lit. Decimal being the smallest cathode it makes sense it would be the most affected. The decimal being lit on one of the other tubes mean it's not dimming related. Every tube reacts slightly different. Also the problem decimal might not actually be pulled solidly to ground which would affect when and how much it lights.
Look into it later when the dust is clearing off the crater.