I’m a Gmail enthusiast like many others. I’ve had the opertunity to use Gmail for a very long time and lately, I’ve noticed that if I Copy/Paste from another location outside Gmail while composing an email, the format suddenly changes to what I had pasted and messes up my email from that paste onwards.
Solution? There is one
Find that icon when composing a Rich Text format email, on the bar along side all the other icons.
See more details on the official Gmail Support page.