To enable the double space period feature on the Samsung keyboard, go to Settings > General Management > Language and input > On-screen keyboard > Samsung Keyboard > Smart typing, and ensure that the “Auto spacing” option is turned on. If the feature is not working, you may need to reset the keyboard settings to default or clear the keyboard’s cache.
EDIT: This (presumably) human, about older versions of the Galaxy:
My Galaxy S25 does the double space inserts a period thing, even with as much predictive text as I can disabled.
Duck Duck Go’s AI suggestion is:
EDIT: This (presumably) human, about older versions of the Galaxy:
https://www.techbone.net/samsung/user-manual/auto-punctuate