I don't know about those Syncros with that plastic gas tank.  But regular Vanagons have a basic float and lever to some kind of potentiometer.....just a means to lift lever and provide voltage output.  The problem is for steel tank 2WD Vanagons (and I guess every car out there!)....the gas tank is not just square and true.  Rather, the tanks are all convoluted to get around things going to and fro.  Thus, the first 1/4 tank will go down quickly on the gauge....but then will settle down.  Just one of the quirks.  But it happens with all fuel powered vehicles.  Annoying, but nothing to worry about.
When you say "now"...what has changed?  Seems to indicate the float/lever system is getting stuck.  Removal of 4WD Vanagon fuel tank to repair fuel sender gives shudders to those that know about how difficult that job is.  At least from what I have read.

    On Sunday, August 28, 2022 at 06:50:03 PM PDT, Chris S <szpejankowski@gmail.com> wrote:  
 
 Same with my Syncro.  I tend to pick pumps with sloped to left parking; it helps. 

Chris. 

Wysłane z iPhone'a

> Wiadomość napisana przez Dan N <dn92610@gmail.com> w dniu 8/28/22, o godz. 16:01:
> 
> This problem could be from when you fill up the gas tank. On my Syncro it
> stops the last 1/4 tank so I have to fill it slowly towards the end. If I
> don't do that the gauge will show 3/4 full when the gas filler nozzle
> clicks off.
> 
>> On Sun, Aug 28, 2022 at 12:17 PM Mark McCulley <mark@markmcculley.com>
>> wrote:
>> 
>> The fuel gauge on my syncro will now only show 3/4 tank when the tank is
>> full. Does this mean the sender needs to be replaced or could there be some
>> other problem?
>> 
>> -Mark
>> ‘87 syncro
>> 
>> Sent from my iPhone