Just wanted to recreate your issue. You're saying that if you misspell a word, so GBoard suggests a corrected word, you can either tap the new word or just hit space. And you're saying that if you do the latter, it ends up as "word_ _" where the underscores are spaces?
I couldn't recreate that with my P3XL, but I did find a setting that might be your issue. Go to GBoard settings>Text Correction, and under corrections there is a toggle for "Autospace after punctuation" that looks like it's a beta setting. Perhaps it's messing something up for you? If that doesn't happen, might be a glitch that can be resolved by clearing the cache or app data.
Let us know if either works!