RRule not setting correct time if dtstart is set - reactjs

Question

I try to make an rrule instance and then set the start date/time (dtstart) but the events get the wrong time.
I initialize a rrule instance with a basis rrule string:
RRULE:FREQ=WEEKLY;INTERVAL=1;WKST=MO;COUNT=20
I set the dtstart date in the rrule.options like so:
rrule.options.dtstart = new Date(Date.UTC(2019, 1, 4, 12, 30, 0))
I console.log rrule.all() to see the generated events. All events now have the current time instead of the time set via dtstart. It should show 12:30 instead
I made this sandbox demonstrating the problem.
Now, when I do exactly the same in rrule.js demo page, I get correct results.
On this demo page, do the following to see what I mean:
These are the results:
As you can see, time of the events is set correctly!
Any advice appreciated!

Answer 1

Found the answer:
If you set distart explicitly in the constructor, then the problem is solved.
Compare these examples:
Example one: gives correct results:
const rrule = new RRule({
freq: RRule.WEEKLY,
interval: 1,
wkst: RRule.MO,
count: 20,
dtstart: new Date(Date.UTC(2019, 1, 4, 12, 30, 0))})
Example two: gives wrong results
const rrule = new RRule({
freq: RRule.WEEKLY,
interval: 1,
wkst: RRule.MO,
count: 20})
rrule.options.dtstart = new Date(Date.UTC(2019, 1, 4, 14, 30, 0));
To me, this looks a bit counter intuitive because setting dtstart should always give the same result whether you set it in the constructor or set it afterwards.