Hope you are doing well and sorry for the trouble. As described the customized tool bar resets to default after some time.
Would you mind sharing the version of the Adobe Acrobat 2020 you are using? To check the version go to Help > About Acrobat and make sure you have the latest version installed. Go to Help > Check for updates. You may also download the patch manually from the link https://www.adobe.com/devnet-docs/acrobatetk/tools/ReleaseNotesDC/index.html and reboot the computer once and check.