can you add relevant part of the custom code along with the question for better response.

Of course, so here's an example:

<?xml version="1.0" encoding="utf-8"?>
<odoo>
<data>
<template id="stock_production_lot_label">
<t t-call="web.html_container">
<t t-foreach="docs" t-as="lot">
<div style="font-size: 4.5em;">
<h1 t-field="lot.product_id.name" />
<p>
<strong>Charge:</strong> <span t-field="lot.name" />
</p>
<t t-if="lot.removal_date">
<p>
<strong>MHD:</strong> <span t-esc="lot.removal_date.strftime('%d.%m.%Y')" />
</p>
</t>
<p>
<strong>Menge gesamt:</strong> <span t-esc="lot.product_qty" /> <span t.esc="lot.product_uom_id.name" />
<span t-field="lot.product_uom_id.name" groups="uom.group_uom"/>
</p>
</div>
</t>
</t>
</template>
</data>
</odoo>

this will result in botched umlauts.