My question is can Humidor liquid containing Propylene Glycol be used instead of normal distilled water?

It actually depends on the liquid. It will work in the beginning. But if only the PG (Propylene Glycol) remains inside, the liquid can drain off accidentally. That applies to all humidifiers. Concerning PG I would recommend to refill with a PG solution only every few months.In our humidors, where we test our liquid for a couple of months, it last very long. Two PG refills a year are fully OK. You can use it more frequently, depending on the actual liquid, but please make sure that the humidifier doesn't accidentally drain off, which would be the case, if there's too much PG inside. (PG is much thicker than water.)

Some retailers and brands recommend using liquids as normal humidor fuel in general. We don't.

My humidor won't reach more than 58%. Can the liquid that I use be the problem here?

You can use a PG solution, but make sure the PG part is not too high. Like with other humidifying substances, the liquid could otherwise drain off later, when only the PG remains and the water is gone. The danger of PG solutions, no matter which substance you use is this: Water evaporates, PG remains and accumulates. After 3-4 refills you will have a very high PG concentration. So, please make sure that you don't use liquids exclusively and still use distilled water most of the times to refill your humidifiers. That refers to Humidor liquids with Propylene Glycol in general.

It's of course a 2 way solution. But please use those Propylene Glycol liquids with great care. That applies to every humidifier brand with which you use them.

We do a lot of tests and I wouldn't recommend to use e.g. xikar's liquid exclusively. It's a good product, but please only use it every 3rd-4th refill.

Please note: Please don’t wipe a humidor out with a Propylene Glycol solution. If it contains Propylenglycol, it will leave an oily stain on the wood. Some 3rd Party Liquids can also affect the function of the humidor crystals inside the humidifer depending on the ingredients. That could also explain the low humidity level.