Any trivial changes to a page break my custom html

I’ve used ChatGPT to write some custom HTML to try to display different content / style content differently on mobile, after having no success with plugins. This code worked just fine, until recently. I edited a random spacer block, to which I had no applicable custom HTML, and I saw that all the custom HTML blocks had stopped being active. All of a sudden, things that were supposed to not appear on mobile were appearing; the text that was supposed to be sized differently on mobile was the desktop size; etc: Aaah!

So I restored my site to the version before I edited the spacer, and everything went back to normal. But now whenever I edit any trivial, unrelated feature of the site, all of this custom HTML becomes inoperative. For example, I can edit a paragraph that has no custom HTML by adding a new word to the text, and all of this HTML will be inoperative when I view the site. Note that deleting the word doesn’t fix the problem; only undoing the change does. What’s going on?

Here’s the code that I’ve added:

<style>
@media only screen and (max-width: 1090px) {<br />
    .desktop-only {<br />
        display: none !important;<br />
    }<br />
}<br />
</style>
<figure class="wp-block-image size-full is-resized desktop-only"><img src="https://staging2.responsiblereportingonai.org/wp-content/uploads/2023/06/halftonecirclecropped1.png" alt="" class="wp-image-274" width="377" height="377"/></figure>

And:

<style>
#only-mobile {<br />
    display: none;<br />
}</p>
<p>@media only screen and (max-width: 1024px) {<br />
    #only-mobile {<br />
        display: block;<br />
    }<br />
}</p>

<style>
@media only screen and (max-width: 1024px) {<br />
    .small-mobile-text {<br />
        font-size: 2rem !important;<br />
    }<br />
}<br />
</style>
<h2 class="wp-block-heading small-mobile-text has-secondary-color has-text-color" style="padding-right:var(--wp--preset--spacing--30);padding-left:var(--wp--preset--spacing--30)"><strong>About us</strong></h2>

I’d be super grateful for any help. I’m totally new to this, and really struggling to create a working site.