I have a revelant site title on my site page, with one of the keywords in the title being: "resizeable". This word have two forms in grammar, and both are correct and used for the same purpose: "resizeable" and "resizable". However, when I test searching for my site in Google typing the title of my page with "resizeable ......." in the phrase to be looked for, Google changes the phrase automatically to "resizable....", indicating that it is searching for a Recommended form of the phrase instead. But this way, my site doesn't show up in the search results at all. However, if I click the link where Google offers: "Search for resizeable.... instead", my site shows up at first place in the search results. Now, you can all have a clue why this is quite annoying, because you can see that my content is so unique, that even if the page has just been posted a few days ago, it is already at 1st place, but visitors won't find it, because most people does't care about (not even notice) that Google automatically changes the phrase to be looked for, and will conclude, that my page simply doesn't exist. Moreover, the situation is even more complicated, because depending on which one of my gadgets I use to test this thing in their borwsers, in some of them the search works this way, in some others, Google doesn't change the phrase to be looked for automatically, but only offers to search for a Recommended form if you click the link on the top. So it is not a good solution for this problem to change the keyword in my page title to the one that Youtube recommends, because you can see on some gadgets this will have an unfavourable result, that is: users who are searching for the form not recommended by Google won't find the page then. Any ideas what would be the solution to make Google rank the page at the same position in either cases when different grammatical variants of the title keyword is used in the search? Because it seems at the moment, that Google doesn't consider grammatical variants of words, when those variants have the same meaning. By the way, you can test this yourself, if you type in Google: "resizeable text area" and "resizable text area". You will see Google doesn't consider that these two variations mean the same, and will only list pages that matches only with one of the forms. Moreover, if the search suggestion option is on (that is usually on by default, and most users don't care about it), that will further complicate the case as I mentioned above.
Have you tried using one form of the word in your title and the other form in your H1? As a former English teacher, I can assure you that probably almost no readers will notice the difference. But this way you'll have both forms of your word in two important places — one form in the title and the other spelling in your H1. Try it and see if it helps. And when you make the change, you might speed up Google's finding the change by 1) if you have a CDN, forcing a recache of that page, then 2) using your Google search console to request a reindexing of that page.
This is why it's so important to actually look at the first page results before choosing a keyword. I would use the variation that Google recommmends/ gets the most searches. You can then use the other variation within your article included in an H2 header and a couple times within the text. No need to stuff it and you will rank for both variations. This is a common trick when dealing with a common misspelled word, just use it a few times within your content.