I did proper assembly from the start where I cleaned and greased them when they were brand new. I’ve never had any issues since. No (cheap) linear bearings come with grease. They only have assembly oil and that is not even a load bearing lubricant; it is a corrosion inhibitor. This is a good thing really, because you need to know exactly what grease your bearings contain. You should never mix greases of any kind. They all have different formulations and will act unpredictably when mixed; often failing in a coagulant that provides no protection from metal on metal contact.

Many cheap printer manufacturers will dab a bit of grease on the rails outside of the bearings when new. This is useless in practice due to the bearing seals. The seals are designed to let a small amount of grease out, but block any old grease from reentering the block itself.

If the blocks were run dry without grease, they are contaminated and need to be cleaned out completely. Likewise if they need service and have unknown grease inside them. If you clean them out to the point they are spotless, and then you manually pack them with a quality grease, you’re unlikely to ever need to service them again for a very long time.

I build my own bicycle wheels and service my bearings and hubs about every 10k miles riding in all weather. I was sloppy with how I serviced bearings for a few years before I really narrowed in on my issues. They must be spotlessly cleaned, without any old grease whatsoever; like clean enough to eat off of them. This is the difference between 2k-4k between problems and 10k+ on a daily ridden bike. Same thing applies here if you want to only do the job once.