The GIF89a standard does not specify looping of an animation. Instead this is implemented through an Application Extension Block. This was first added in Netscape Navigator 2.
The standard specifies that Data Blocks (an Application Extension Block is a Data Block) can appear in any order; Check the grammar at the bottom of specification.
There is a guide asserting that the Netscape block (NAB) must follow directly after the global colour table, but it also references Netscape Navigator 2.0beta4 which today is definitely outdated. This then probably deals with implementation details specific to that browser.
This seems to favour @couet’s statement that this is a problem in the LibreOffice Impress implementation 
Cheers,
Kim