Based on some recent Reddit complaints and subsequent reports, the spell check feature on Google's popular keyboard app for android - Gboard - appears to have been broken for quite some time now. Specifically, that is the spell check feature which underlines misspelled words in red. It's similar to how the feature works in some other keyboards or in some native apps but the feature appears to have been malfunctioning since November if some of the reports are to be believed. Meanwhile, Google has at least known about the problem since the January, when the company apparently first responded to the problem. The company's mods reportedly took Google's support forums but simply stated that moderators were passing the problem on to the development team. Despite several updates over the past few months, however, the problem seems to still not be fixed.
In the meantime, it's extremely difficult to pin down where the problem might be stemming from. One common theme in the reports of the problem seems to link it to installation or pre-installation of Android 8.0 or newer. That's devices that were either updated to or shipped with Oreo out of the box. However, reports aren't necessarily confined to that firmware update at all, although it might still be an issue at the OS level. Since Google has still not officially addressed the issue or offered a fix to the problem, there's ample room there to speculate but there's no way to know for sure. There's also no way to know when or if a fix is on the way.
Bearing all of that in mind, it may be a good idea for those who require accurate spelling for whatever reason to switch to another keyboard. At the very least, because sometimes spelling is more important than others, it might be a good idea until a fix appears in Gboard's update log. Several other apps in the category have similar features to prevent misspelled words from cropping up in emails or text messages. Grammarly, for example, goes well beyond spell checking and provides suggestions for grammatical errors and writer's voice corrections. Best of all, it does that in real time.