Is it ok to have paragraphs that have only one sentence in a formal paper?

The Wikipedia article on paragraph says a para could contain one or more sentences. The very purpose of grouping content into paragraphs is to organise similar thoughts into one unit. Too many small or one-sentence paragraphs will affect the cohesiveness of your content.

But a one-sentence para may be fine at the end of a section in a paper if it adequately summarises the content within. (Like this, perhaps!)

If the result reads like a newspaper, I would say, no, it's not alright.

If it reads like Tractatus Logico-Philosophicus, then you are probably doing something right.

Unless you are an exceptional writer, my advice is to follow the accepted conventions of your field, which probably includes block paragraphs.

A longer paragraph allows one to build up an idea and explore it more thoroughly. The first sentence introduces the idea. Subsequent sentences explore it in more detail. There should be a logical connection between sentences, and sentences should ideally vary in length. Developing good style is matter of practice and reflection. And there is more to good style than the length of a paragraph.