I tested this myself and was unable to replicate the issue. I do, however, have a few questions to help narrow down the problem.
1. Which products work, and which ones don't?
2. Is there a difference between those products? (i.e. do some products require additional apps to add text)
3. Does this happen on both mobile and desktop?
4. Are you using a Shopify free theme, or a third-party theme?
With a bit more context, I'll be able to help diagnose the issue, and help point you in the right direction to sourcing a solution.