Mobile Version sometimes showing on Desktop

Hello,
Different users are complaining that they sometimes see the mobile version on a normal desktop (big screens)
This happens only sometimes for no apparent reason.
Do you have an idea? Or could this have to do something with the changes you made in May (there was a mobile error)?
Thank you

Hello Calin,

I have not activated the Cache Plugin (WP Super Cache)
But instead have some code in the htaccess for this.
Could this be the issue? My site is very fast ever since, especially on mobile. I would prefer to keep this code

# BEGIN Expire headers

<ifModule mod_expires.c>
ExpiresActive On
ExpiresDefault “access plus 5 seconds”
ExpiresByType image/x-icon “access plus 604800 seconds”
ExpiresByType image/jpeg “access plus 604800 seconds”
ExpiresByType image/png “access plus 604800 seconds”
ExpiresByType image/gif “access plus 604800 seconds”
ExpiresByType application/x-shockwave-flash “access plus 604800 seconds”
ExpiresByType text/css “access plus 604800 seconds”
ExpiresByType text/javascript “access plus 604800 seconds”
ExpiresByType application/javascript “access plus 604800 seconds”
ExpiresByType application/x-javascript “access plus 604800 seconds”
ExpiresByType font/truetype “access plus 604800 seconds”
ExpiresByType font/opentype “access plus 604800 seconds”
ExpiresByType application/x-font-woff “access plus 604800 seconds”
ExpiresByType image/svg+xml “access plus 604800 seconds”
ExpiresByType application/vnd.ms-fontobject “access plus 604800 seconds”
ExpiresByType text/html “access plus 600 seconds”
ExpiresByType application/xhtml+xml “access plus 600 seconds”
</ifModule>
# END Expire headers

<IfModule mod_deflate.c>
<filesMatch “\.(js|css|woff|html|php)$”>
SetOutputFilter DEFLATE
</filesMatch>
</IfModule>

<IfModule mod_headers.c>
<FilesMatch “\.(ttf|ttc|otf|eot|woff|woff2|font.css|css|js)$”>
Header set Access-Control-Allow-Origin “*”
</FilesMatch>
</IfModule>

Thanks!
Britt