There is no grammar rule that I’m aware of stating that all the verbs in one sentence have to be in the same tense. Naturally, that will often be the case, but it simply depends on what you want to talk about.
I think if you take your two sentences apart and turn them into separate, shorter sentences, you can see more easily that the various verb tenses are not only logical but also correct:
Numerous collections of short stories include works by Isaak Bashevis Singer.
[color=blue](This sentence states a fact that is true in general, therefore you need the simple present tense. Singer’s work is generally available in various collections of short stories. It’s also possible that Singer’s work will be published in various new collections in the future.)
Isaak Bashevis Singer continued to write primarily in Yiddish, despite living in the United States for more than fifty years.
[color=blue](All of Singer’s work was written in the past. Because Singer is dead, you definitely cannot say “has continued” or “continues” to write.)
There is an ancient concept that states that the sun revolves around Earth.
[color=blue](The concept still exists and what it states is unchanged. The only thing that is different is that nobody has agreed with it for a very long time.)
This concept was questioned by Copernicus in the sixteenth century.
[color=blue](This refers very specifically to an event in the finished past.)
Does that help?