Can I forget a topic in mathematics even after doing large amounts of practice problems and understanding the topic fully?

Of course you can forget a topic — this is what normally happens.

Moreover, this is very likely to happen if you had no chance to use this particular topic outside of the course where it was introduced and where you practiced it.

The best way to consolidate your mathematical skills is to use them at the next step of learning mathematics.

I have been teaching mathematics for 45 years, and I have seen, again and again, students, who have finally understood the long division of integers when they have encountered and learnt the long division of polynomials. Or understood algebra of rational functions only when they learned how to use them in integration of trigonometric functions, which, at the first glance, have absolutely nothing in common with rational functions.

This is one of the neglected and unmentionable laws of mathematics education:

The Law of Excessive Learning: to be able to operate at a certain level of mathematics, you have to learn at at the next level.

This law is fully supported by the practice of employment: businesses hire university mathematics graduates for jobs which need confident mastery of school level maths; for jobs where some understanding of undergraduate mathematics is needed, they hire people with PhD or at least MSc in mathematics intensive disciplines.

And, finally, another best kept secret of education: forgetting is an integral and critically important part of learning. Forgetting is removal of scaffolding from a completed building. What so frequently happens in school and university level mathematics education — students get no building, they are fraudulently sold the scaffolding.