i have been groving pineapples planted from suckers. Two of three have been growing fine, but now are developing brovn soft areas on the leaves, but not at the tip or the base. They were planted in potting soil with some ceramsite on the bottom of the pot for drainage. I water them them when the soil gets dry. Two weeks ago i started watering them with added fluid hoseplant fertilyser. I add the fertilizer according to yhe instructions. They are places next to a window facing east-south side (living in Denmark) . I don't see any mealy bugs ar any other pests. Maybe I over water them?
I also grow pineapples in a northern climate.
It is important to note that pineapples feed and drink from their crown, not from their roots. The roots are to almost all exclusion used to anchor the plant only. When outside in the summer months, the evening and morning dew is enough for much of the plants’ water requirements and rain usually takes care of the rest, but indoor they do not get dew nor rain. You need to water the plant by pouring water into the crown (the center) of the plant. Do not use chemical fertilizer, and if you use natural fertilizer such as fish emulsion, make it VERY weak. They don’t need or want much because they draw most of their nutrients directly from the air.