Header JS issues with WP 5.5 update

[alexmustin]

Describe the bug
New issues with the header introduced with the latest WP 5.5 release:

• Sticky Header not behaving properly on scroll.
• Mobile Nav Menu Button disappears when screen is 1023px or narrower.

To Reproduce
Steps to reproduce the behavior:

1. Update WP to 5.5
2. Ensure 'sticky header' option is enabled in Customizer
3. Scroll down on a page, see the Sticky Header issue
4. Make the browser window very narrow, see the Mobile Nav Menu Button issue

Expected behavior
Sticky Header should behave as expected.
Mobile Nav Menu Button should appear when screen is 1023px or narrower.

Screenshots
N/A

Desktop (please complete the following information):
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Smartphone (please complete the following information):
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Additional context
Could be due to WP deprecating support for jQuery Migrate:
https://www.searchenginejournal.com/wordpress-5-5-issues/377851/