Button disappears

Questions
1

i have s strange problem. on my homepage i have a simple button

				<!-- Scrollable page content -->
				<div class="page-content hide-toolbar-on-scroll ptr-content">
					<div class="ptr-preloader">
						<div class="preloader"></div>
						<div class="ptr-arrow"></div>
					</div>
					<div class="block-title">
						<p>Tickets <i class="f7-icons">ticket</i> auf Gültigkeit überprüfen.</p>
					</div>
					<div class="block block-strong">
						Bitte den <i class="f7-icons">qrcode</i> scannen...
						<p style="text-align: center"><button id="startScan" class="button button-large button-fill inaktiv">Ticket scannen</button></p>
					</div>
				</div>

one of our customers told me that he can’t always see the button on homepage. It disappears. He can open the app several times and then the button get’s displayed.
how can i debug this?

i only set the button #startScan to inactive if the app is not registered

	$$('#startScan').addClass('inaktiv');
	$$('#startScan').off('click');

after registering i enable the button

	if ( $$("#startScan").hasClass('inaktiv') ) { 
		$$('#startScan').removeClass('inaktiv');
		$$('#startScan').on('click', function () {
			mainView.router.navigate('/scanResult/');
		});	
		// console.log('ScanButton: OK: now enabled')
	}

i tried it on another iphone, i tried it on my iphone but i don’t see this. latest ios 16.02 on iphone 12 or 13
Framework7 5.1.3