When you initiate emails from your iPhone (iOS) it results in plain text format on the receiving end.
By default, an iPhone will always send new messages in Plain Text unless any formatting has been applied to the text.
If formatting is applied to the text, such as making the text bold or italic, then it will send the message in HTML.
Resolution - For Office 365
You can change to HTML when the email is sent from iOS devices.
First, Open 'Preferences' menu, select 'Integration' tab. click 'Start Integrate' to connect to your Office 365.
Then select 'Convert Plain Text emails to HTML for iOS devices' and sent an email to an external contact:
WORKAROUND - For Exchange On-premise
You can also add a space which you format as bold.
With these change, all emails sent from your iPhone that include this signature will be sent in HTML.
On the iPhone tap Settings > Mail, Contacts, Calendars and scroll down to the Signature section.
Tap Signature and then insert a space.
Select the space and then select the B/U button.
Make the text bold or italic and then tap Mail (top left) to save the settings.