Your post has been moved to a dedicated topic.
Please avoid adding your own request for Support to another member’s topic. A separate topic will allow others to help with your specific issue(s) without any potential confusion.
You have provided your system information. Thank you for that. I’m sure someone will help when they are able.
In the meantime, please rename the title to something that better explains the problem you are experiencing. 
Regards.
Your description didn’t seem immediately clear, but with fresh eyes I can see that it might only be a matter of configuration.
If I now understand the problem correctly, the easiest cure might be to edit the /etc/default/grub file directly;
1. To have the Grub menu display and the last booted OS set as Grub’s default boot selection, ensure that these lines are uncommented; change as needed, but this works for me:
GRUB_DEFAULT=saved
GRUB_TIMEOUT=5
GRUB_TIMEOUT_STYLE=menu
GRUB_SAVEDEFAULT=true
2. And then, uncomment this line (leaving it exactly as shown) to ensure other loaders are detected;
GRUB_DISABLE_OS_PROBER=false
Close and save.
3. Next, update the Grub configuration:
sudo update-grub
4. Reboot.
Welcome to the Manjaro community
As a new or infrequent forum user, please take some time to familiarise yourself with Forum requirements, and the many ways to use the forum to your benefit.
To that end, links are provided (below) - Please use them.
Be prepared to provide outputs from various commands when asked. It’s equally important to provide as much actionable information as possible in your first post, rather than simply indicating there is a problem.
Waiting for someone to blindly ask questions is also counter-productive – typically, nobody has a 
at their disposal – Instead, please help others to make informed suggestions, based on information you provide.
Update Announcements
The Update Announcements contain important information and a Known Issues and Solutions section that should generally be checked before posting a request for support.
System Information
While information from *-fetch type apps might be fine for someone wishing to buy your computer, for Support purposes it’s better to ask your system directly;
Output of the inxi command (with appropriate parameters, and formatted according to forum guidelines) will generate information useful for those wishing to help:
inxi --filter --verbosity=8
or the short form (preferred):
inxi -zv8
- inxi man pages (Arch Manual)