I experienced a similar problem. The size slider bar actually changed the line height instead of adjusting the font size. In my case this happened with text that had been pasted into the text editor from another source. Text that had been entered from the keyboard was no problem.
Solution; when copying a block of text from anywhere, I now past in into a basic text editor, recopy and then past into Elementor = perfect !!
I hope my experience is beneficial to others.
I love Elementor ??
